Increases in the cost of petroleum and concerns about future shortages has led to increased interest in other carbonaceous energy resources, such as coal, tar sands, shale and the mixtures thereof. Coal is the most important of these alternative resources for reasons including the fact that vast, easily accessible coal deposits exist in several parts of the world, and the other resources contain a much higher proportion of mineral matter and a lower carbon content. Various processes have been proposed for converting such materials to liquid and gaseous fuel products including gasoline, diesel fuel, aviation fuel and heating oils, and, in some cases, to other products such as lubricants and chemicals.
A number of problems have hampered widespread use of coal and other solid fossil energy sources that include the relatively low thermal efficiency of indirect coal-to-liquids (CTL) conversion methods, such as Fischer Tropsch (FT) synthesis and methanol-to-liquids (MTL) conversion. The conversion of coal, which has a H/C ratio of approximately 1:1, to hydrocarbon products, such as fuels that have H/C ratio of something greater than 2:1 results in at least half of the carbon in the coal being converted to CO2, and thereby wasted. Additionally, the fact that a large amount of greenhouse gas (GHG), particularly in the form of CO2, is emitted as a waste product in the conversion of coal to useful products has caused CTL processes to be disfavored by many from an environmental point of view.
It has been proposed to at least partially overcome the GHG problem by capturing and sequestering the carbon dioxide by re-injecting it into subterranean formations. Such an arrangement has the disadvantages of being expensive, requiring the availability of appropriate subterranean formations somewhere in the vicinity of the conversion facility, concerns about the subsequent escape into the atmosphere of the carbon dioxide, and the waste of the energy potential of the carbon content of the carbon dioxide.
The conversion of coal to valuable liquid products by indirect methods involves syngas generation. Syngas, a mixture of mainly carbon monoxide and hydrogen, can be used as a feedstock for producing a wide range of products, including liquid fuels, methanol, acetic acid, dimethyl ether, oxo alcohols, isocyanates, etc. Syngas can be generated from carbonaceous materials, such as coal, or from biomass, via gasification. It is possible to produce syngas from coal with a H2/CO ratio that is about 0.5 to about 1 using commercially available gasifiers. However, when used to produce liquid products by FT synthesis or MTL conversion, syngas with a H2/CO ratio of about 2 is desired. The H2/CO ratio of the coal produced syngas can be raised to the desired range with the water gas-shift reaction. That, however, results in large carbon dioxide emissions.
A report to the National Energy Technology Laboratory entitled “Increasing Security and Reducing Carbon Admissions of the US Transportation Sector: A Transformational Role for Coal with Biomass,” DOE/NETL2007/1298, proposes reducing the amount of CO2 emissions generated by gasifying coal for FT synthesis by about 20% by co-gasifying the coal with 10-15% biomass, such as a woody biomass, switchgrass, or corn stover, which have a higher H2/CO ratio. A number of problems exist with the proposed method, however. The thermal efficiency of the process is relatively low because of the energy required to gasifying coal and biomass, typically by partial oxidation, and the use of indirect FT synthesis. The required land area used to produce the biomass, and the proximity thereof to the ICTL facility, also limits the amount of biomass that can be economically employed to a maximum of about 5000 to 10,000 barrel equivalents per day. Additionally, the biomass is a substantially more expensive source of syngas than mineral carbonaceous sources such as coal, thereby adding to the product cost.
Direct coal liquefaction (DCL) methods have been developed for liquefying carbonaceous materials such as coal that have advantages in many applications to conversion by FT synthesis, including substantially higher thermal efficiency and somewhat lower CO2 emissions. Such direct liquefaction methods typically involve heating the carbonaceous material and a solvent in a hydrogen containing atmosphere to a temperature in the range of about 775° to 850° F. in the presence of a catalyst, typically very finely divided iron or molybdenum or mixtures thereof, to break down the coal structure into free radicals that are quenched to produce liquid products. Hybrid coal liquefaction systems involving both direct liquefaction and FT synthesis, or direct liquefaction and biomass conversion have been proposed in which the FT synthesis or biomass conversion provides additional hydrogen for the direct liquefaction, thereby reducing carbon dioxide emissions. Hybrid coal liquefaction systems involving all three of direct liquefaction, FT synthesis, and biomass conversion have also been proposed. None of these proposed arrangements, however, achieve the combination of thermal efficiency, low cost and substantially reduced GHG emissions that would be required for them to be economically and environmentally attractive. There remains an important need for economical coal and biomass to liquids conversion processes with reduced carbon dioxide emissions and efficient use of carbon resources.